The present invention relates to a ferritic stainless steel alloy. More specifically this invention relates to an alloy suitable for use in industrial and other heating applications, such as electric heating elements in diffusion furnaces for the production of semiconductors and similar applications having special demands regarding ultra low content of impurities, more specifically an ultra low content of copper.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicant reserves the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Heat treatment is a typical operation in many industries, for example in the manufacturing of semiconductor wafers. During such process semiconductor wafers are heated in furnaces to temperatures of 700xc2x0 C. to 1250xc2x0 C. in order to alter the properties or composition of the surface of said semiconductor wafers. For example, heat treatment in controlled gaseous atmosphere allows certain dopant elements to migrate into the structure of the semiconductor material. A controlled environment within a diffusion furnace brings about a predictable result. Problems can occur in the control of the environment within the diffusion furnace. Certain harmful impurities tend to be introduced into the furnace, for example, by diffusion of alloying elements or impurities from the heating elements. These impurities can then find their way into the semiconductor wafers. Adverse effects of those harmful impurities show a tendency to increase with time of use of the furnace/tube. This has been a problem for this kind of application for a long time (see, e.g.xe2x80x94U.S. Pat. No. 4,347,431).
It has been found that a yield for the production of special types of semiconductors is limited by Cu-contamination during the production of said semiconductor wafers. Copper has been identified as one of the most harmful impurities. Heating elements in the diffusion furnace have been identified as a source for this Cu-contamination during a long range of different tests.
One problem that occurs in connection with the measurement of contents of elements that usually occur as impurities in the manufacture of alloys used for heating elements, is that those low contents of elements and/or impurities can not be measured with a satisfying accuracy. Special test methods, as described in detail later, had to be used, even in order to show the advantages of the alloy of the present invention.
Ferritic stainless steel alloys, usually referred to as FeCrAl-alloys, are resistant to thermal cyclic oxidation at elevated temperatures and suitable for forming a protective oxide layer such as, i.e. an adherent layer/scale of aluminum on the surface of the alloy after heat treatment. This oxide layer/scale is considered to be one of the most stable protecting oxides/layers on the surface of an alloy of said type, having low oxidation rates at high temperatures and at the same time resist to cyclic thermal stress during long periods of time. It has been shown that this type of alloy can advantageously be used in applications such as for example exhaust emission control systems for the automotive industry, applications with high demands regarding resistance for high temperature induced corrosion, such as turbine rotors and industrial and other heating applications, such as electrical heating or resistance heating elements.
A limiting factor for the lifetime of this type of alloys is the content of aluminum. During the use of parts manufactured of these alloys and their exposure to cyclic thermal stress, the aluminum migrates to the surface, forms alumina and will be consumed after a certain period of time. It is known that a range of other elements, such as rare earth metals, have an effect on the rate of consumption of aluminum from the alloy and hence limits the lifetime.
Another limiting factor is the different rate of elongation between the oxide-layer and the surface of the alloy. The core alloy of, for example, a wire, expands its volume in a considerably higher amount than the oxide scale that covers this core. The oxide scale is hard and brittle and withstands the forces that core exerts until cracks in this scale and spallation of oxide scale occurs. These cracks will be sealed by newly formed oxide under said heating. This healing process of the oxide consumes the aluminum from the alloy core. This effect is a typical restriction for the use of said alloy for heating applications.
It is an object of the invention to provide an iron-chromium-aluminum alloy, a so-called FeCrAl alloy with for the use in industrial and other heating applications. More specifically for the use as electrical heating element in, for example, diffusion furnaces used in the electronics industry, i.e. in diffusion furnaces for the manufacture of semiconductor wafers for the use in applications with high demands to the purity of the semiconductors regarding the content of impurities, especially the content of copper.
Another object of the present invention is the considerable longer life time of the electric heating element, since the alloy of the invention appears to show lower Al depletion rate and smaller amount of elongation than known alloys for the above mentioned purpose.
According to one aspect, the present invention provides a ferritic stainless steel alloy comprising, in weight %, less than 0.02% carbon; xe2x89xa60.5% silicon; xe2x89xa60.2% manganese; 10.0-40.0% chromium; xe2x89xa60.6% nickel; xe2x89xa60.01% copper; 2.0-10.0% aluminum; one or more of Sc, Y, La, Ce, Ti, Zr, Hf, V, Nb and Ta in an amount of 0.1-1.0; remainder iron and unavoidable impurities.
According to another aspect, the present invention provides an electrical heating element containing, at least in part a ferritic stainless steel alloy comprising, in weight %, less than 0.02% carbon; xe2x89xa60.5% silicon; xe2x89xa60.2% manganese; 10.0-40.0% chromium; xe2x89xa60.6% nickel; xe2x89xa60.01% copper; 2.0-10.0% aluminum; one or more of Sc, Y, La, Ce, Ti, Zr, Hf, V, Nb and Ta in an amount of 0.1-1.0; remainder iron and unavoidable impurities.
According to yet another aspect, the present invention provides a diffusion furnace comprising a heating element formed from an alloy according to the principles of the present invention.